In a conventional OTEC system, many components are located below the water line. One such component is the working fluid condenser heat exchanger. Due to net positive suction head required, the working fluid pumps must be located below the condensers, and so the working fluid pumps must also be located below the water line.
Commercial Off The Shelf (COTS) working fluid pumps are not available for water submergence. As a result, the working fluid pump(s) must be located inside a pressure vessel to isolate the pump(s) from the ocean environment and pressure differences. This complicates heat dissipation. Heat must be dissipated from the motor to keep the major working components, such as motor windings, seals, bearings and lubricant, cool.
The inefficiency of the motor (nominally 5%) will be directly converted to mechanical work in the form of heat. This heat must be extracted at the rate it is produced. Furthermore, the motor temperature cannot exceed a certain temperature, generally 40 degrees Celsius. One solution is to use the sea water surrounding the pressure vessel as the heat sink. Unfortunately, the pressure vessel may not account for enough surface area to rely on air convection on the inside of the pressure vessel and natural sea water convection on the outside of the pressure vessel. Even if water is forced around the vessel to create forced convection while air is circulated to create forced convection on the inside of the pressure vessel, enough heat may not dissipate quickly enough to sufficiently cool the motor.
Another option is to use sea water that is pumped through a heat exchanger located inside the pressure vessel or located outside the pressure vessel in order to cool the motor. Unfortunately, this option requires a sea water inlet and a sea water outlet (in the case of an internal heat exchanger), or an additional working fluid inlet and outlet to the heat exchanger (in the case of an external heat exchanger), a sea water filter (failure point), an additional pump (failure point), additional seals (failure points), and uses sea water directly, which is highly corrosive (increases failure potential).